In order to ensure a high degree of stability and grid quality in the supply and transmission grids for electrical energy, grid operators are to an increasing extent demanding support from energy generators. This relates not only to conventional power plants, but to an increasing extent also to wind turbines or wind farms consisting of a plurality of wind turbines. In principle, it is the case that wind farms are connected to the grid via so-called tap-changing transformers in precisely the same way as conventional power plants. Tap-changing transformers are transformers which are provided with a tap changer for matching to a changing voltage level on the grid. Since switchover of the tap changer results in change in voltage, conventional power plants respond with a short-term change in the reactive power output, to be precise owing to the synchronous generators typically used in said power plants and the specific characteristics of the said synchronous generators. In general, such a reactive power output is definitely desirable in the case of a real change in voltage, but not when it is merely a switchover operation of the tap-changing transformer. This problem with the undesired output of reactive power as a result of switching operations at the tap changer is prevalent to an increased extent in the case of wind turbines since wind turbines or wind farms consisting of a plurality of wind turbines are typically connected to the grid via a tap changer which is tapped more coarsely in comparison with a tap changer of a conventional power plant.
Relevant grid operator guidelines make provision for the fact that the reactive power output of wind turbines takes place with a dead band. In this case, a dead band is understood to mean a tolerance range for the voltage around the rated value for the voltage. Since wind turbines do not have the reactive power output behavior of conventional power plants which has an intrinsically stabilizing effect owing to the type of generator construction typically used in wind turbines, the application of the dead band can result in an undesired reduction in the voltage stability.